PASCAL: Files

USING A FILE IN PASCAL
Files are referred to in Pascal programs by the use of filenames. You have already used two default filenames, input and output. These are associated with the keyboard and console screen. To derive data from another source, it must be specified in the program heading, eg,


	program  FILE_OUTPUT( input, fdata );

This informs Pascal that you will be using a file called fdata. Within the variable declaration section, the file type is declared, eg


	var  fdata : file of char;

This declares the file fdata as consisting of a sequence of characters. Pascal provides a standard definition called TEXT for this, so the following statement is identical,


	var   fdata : TEXT;

BASIC FILE OPERATIONS
Once the file is known to the program, the operations which may be performed are,

The file is prepared for use by RESET or REWRITE
Information is read or written using READ or WRITE
The file is then closed by using CLOSE

PREPARING A FILE READY FOR USE
The two commands for preparing a file ready for use in a program are RESET and REWRITE. Both procedures use the name of the file variable you want to work with. They also accept a string which is then associated with the file variable, eg


	var  filename : string[15];

	     readln( filename );

RESET ( fdata, filename );
This prepares the file specified by filename for reading. All reading operations are performed using fdata.
REWRITE ( fdata, filename );
This prepares the file specified by filename for writing. All write operations are performed using fdata. If the file already exists, it is re-created, and all existing information lost!

READING AND WRITING TO A FILE OF TYPE TEXT
The procedures READ and WRITE can be used. These procedures also accept the name of the file, eg,


	writeln( fdata, 'Hello there. How are you?');

writes the text string to the file fdata rather than the standard output device.

Turbo Pascal users must use the assign statement, as only one parameter may be supplied to either reset or rewrite.


	assign( fdata, filename );
	reset(  fdata );
	rewrite( fdata );

CLOSING A FILE
When all operations are finished, the file is closed. This is necessary, as it informs the program that you have finished with the file. The program releases any memory associated with the file, ensuring its (the files) integrity.


	CLOSE( fdata );         {closes file associated with fdata}

Once a file has been closed, no further file operations on that file are possible (unless you prepare it again).